The present invention generally pertains to fission chamber detection systems for monitoring neutron flux in a nuclear reactor, and is particularly directed to and providing high neutron signal sensitivity in a hostile environment.
A fission chamber detector is a type of neutron detector that is preferred for use in neutron flux monitoring systems because it has been proven to have a longer life and to be more reliable than other types of neutron detectors.
In a typical prior art fission chamber detector system for monitoring neutron flux in a nuclear reactor, a number of fission chambers are located inside a biological shield that surrounds the reactor core. Neutron signals produced in response to the detection of neutrons are transferred over conductors, such as coaxial cables to a preamplifier unit located inside a containment vessel for the reactor. The preamplifier unit amplifies the neutron signals for enabling further transmission via coaxial cables.
In prior art systems, the preamplifier units are located within the containment vessel for the nuclear reactor because in such prior art systems, the quality of the neutron signals would be so much diminished by electrical noise, attenuation, and signal reflection if the preamplifier units were located more than one hundred feet (thirty meters) from the neutron chambers that the sensitivity of the system would be impaired. The location of the preamplifier units within the containment vessel makes the preamplifier units susceptible to being rendered inoperable in the event that they are subjected to a hostile environment such as exists when the reactor suffers a loss of coolant accident. In the event of such an accident, the environment within the containment vessel is severely changed. Steam, boric acid, caustic sprays and other contaminants that are adverse to electrical circuits permeate the air, and the temperature and the air pressure increase to such an extent that preamplifier units in conventional containers would not withstand the increased temperature and would be damaged by such contaminants as penetrated the container under the conditions of increased pressure. Also the radiation level would increase to make the preamplifier units inoperative from the radiation damage. Yet, it is particularly important that neutron flux within the biological shield be monitored during and following a loss-of-coolant accident. This would require preamplifier units located within the containment vessel to be shielded from high radiation and temperature and to have containers that can withstand high pressure and be impermeable to contaminants. It is impractical and very expensive to meet this requirement.